For those that haven’t encountered the term before, a Tweetup is a real-life get together between people who use Twitter. Beyond that, the rules are flexible: Tweetups can be large events or small gatherings, can involve grabbing a few drinks or just socializing for a bit, and can be planned for in advance or spontaneous. Founder Jeff Morin says that while there are plenty of sites that cater to traditional events, like birthday and BBQs, the Tweetup niche is underserved.

Setting up an event with Tweetvite will be familiar to anyone who has used an event site like Anyvite or Evite. To get started, you enter the name of your event, the location, who is hosting it, and other essential information. But the site includes a few attributes that you won’t find anywhere else: it asks you to designate a hashtag for the event, as well as a custom shortened URL. The site also makes it easy to Tweet out your event, or share it with other services like Facebook and MySpace. Another big difference from traditional events sites is the fact that Tweetvite offers a directory of upcoming Tweetups (given the nature in which they’re announced, they’re generally open to the public).

Once you’ve created your event, you can use the site’s control panel to monitor for any tweets containing your hashtag and see how many people have viewed your page and RSVP’d. The site also offers a widget that you can embed on your blog to inform visitors of your upcoming tweetup.

Tweetvite looks great, with a very polished interface and a streamlined event creation process that only takes a minute or two. At this point the biggest question in my mind is how many people actually throw Tweetups — they may be becoming increasingly popular but are nowhere near as common as traditional events are, so it may be tough to build a business around this niche. That said, Twitter is obviously still in its infancy, so the number of Tweetups may grow rapidly over the next few years.

The twenty one startups from Y Combinator’s summer session are presenting their ideas and creations to investors in Boston this afternoon. Below are descriptions of the nine startups we haven’t covered and who don’t wish to remain in stealth mode any longer. See our prior coverage of Posterous, Anyvite, ididwork, Popcuts, and Slinkset – all of which are part of this batch and have launched already.

TicketStumbler

TicketStumbler can be described as Kayak for sports tickets. It aggregates tickets from sites like StubHub and RazorGator, making them searchable by keyword and allowing for the filtering of results by maximum price, quantity available, provider, etc. The site is live, fast, and gets extra points for not spelling “stumbler” without the “e”.

People and Pages

While yet to launch, the founders of People and Pages describe their service as “a better Google Groups”, although the screenshots show that it’s part WYSIWYG website creation tool as well, making it competitive with Google Sites, Weebly (also a Y Combinator startup), and others. Group organizers can use People and Pages to manage email lists and publish to the web in one place.

MeetCast

MeetCast is a WebEx and GoToMeeting competitor (yes, another one) that is marketing itself on ease of use (no downloads) and playback (all conferences are saved and indexed for later viewing). The founders draw comparisons to Tokbox for its simplicity.

CO2Stats

For a flat monthly fee, CO2Stats will measure the overall electricity usage of websites and then automatically buy renewable energy certificates for them to offset their effective emissions. Founded by academics from Harvard and Yale, CO2Stats has already turned a profit by signing up 2,500+ sites in over 25 countries. See our review from earlier today.

Youlicit

Youlicit is a service prepping for relaunch that will generate Mahalo-like search guides by scouring the web for user generated content and compiling it into topics algorithmically instead of relying on human editors. These search guides themselves are intended to show up highly in the results of more traditional search engines like Google.

Job Alchemist

Job Alchemist is the parent company of two online services: Startuply, a job site for tech startups that we covered last month, and a new job affiliate network called JobSyndicate that launches today. Publishers can place JobSyndicate’s widgets on their sites and earn half the bounty set by employers when visitors click through and get hired.

Frogmetrics

Frogmetrics isn’t a pure web venture: the company wants to place touch screens in restaurants, stores, and other brick and mortar establishments that can be used to collect customer feedback on the spot. The devices ask customers a few questions at the point of sale about their experience and can collect contact information about customers to generate leads. The information gathered across physical locations is aggregated and analyzed for trends and other statistics.

Snipd

Snipd appears to be another web annotation service, one that allows users to “snip” page content such as images, videos, and text to share them with others and save for later. These snips are also used to generate so-called heat maps of pages that help users find the best content on a page. The service has yet to launch.

BackType

BackType is a search engine for comments that crawls the internet for blogs and indexes their user generated content regardless of the platform (WordPress, Moveable Type, etc). These comments are not only keyword searchable but can be followed by author, allowing you to keep track of what your friends are saying online.

It’s no secret that Evite sucks. It’s cluttered, it unnecessarily withholds information from email messages in order to drive page hits, and it looks like it hasn’t seen a facelift in about a decade.

The latest company to take on the much-maligned site is Anyvite, a Y Combinator-funded startup that launches today. Anyvite has taken a very streamlined approach to the invite process, requiring a minimal amount of information to get an event setup. And replying to an invite is even easier – you can actually RSVP from the email itself (take that Evite).

One of Anyvite’s most valuable features is its integration with mobile devices. The site features a robust mobile site (http://m.anyvite.com) that makes it easy to create and edit events on the go. Perhaps even more useful is Anyvite’s integration with SMS, which allows users to immediately get updated on a change in plans. You can also send mass SMS messages to anyone else that has been invited to an event, which makes it sort of like a Twitter for groups.

Anyvite president Jeff Morin says that these mobile features are designed to make the site better suited for impromptu get-togethers. You can create general events, like “Friday Bar Night”, and let your invitees discuss the details over SMS and email. Then, once you’ve figured everything out, you can send out an updated invitation with the details. In practice everything worked as advertised – it took less than a minute to create an event and the very web-2.0 interface was easy to navigate. Maybe this will be the site that finally gets people to stop using Evite.

The guys behind Anyvite launched a very similar site called Coordinatr last February (the FAQ section looks cut-and-pasted). Morin says that Coordinatr was primarily used as a technology test with only a few, basic features (it lacked the SMS functionality entirely). Since then the team has gradually improved the site, but after becoming affiliated with Y Combinator they decided to rebrand and relaunch as Anyvite.

Besides Evite, Anyvite will see competition from a number of similar invite services that include Socializr, MyPunchBowl, and MadeIt.