These services are very popular, particularly with employees and students who are unable to download and use normal IM clients on their computers.
Kool IM takes features from both Meebo and eBuddy. Like eBuddy, Kool IM does chat through a browser pop up window (Meebe embeds it on the page, although it can be dragged). I prefer the Meebo approach, but it’s a matter of personal preference. Kool IM also allows (but does not require) users to create accounts, so future logins to multiple IM services do not require separate logins to each service. Meebo has this feature, eBuddy does not (but says they will soon).
In addition to Silicon Valley based Meebo and Amsterdam based eBuddy, ILoveIM, Germany based Mabber and other competitors are out there vying for the same user base. Luckily, users tend to rack up a lot of time (if not page views) with the service, allowing the companies to generate reasonable revenue from advertising – eBuddy claims to be profitable.