I've been using the free version GMail for Domains since it was released, and I must say that I've had nothing but a positive experience. Tons of space, the best spam filtering available, and the wonderful Gmail interface. I highly recommend it!

I use Gmail for every domain I use email with. It is absolutely the best "web-version" of an email client anyone can ask for. When I went looking for an alternative for SquirrelMail to install on my domain, I found an app called RoundCube, which is actually quite nice. I used it for a while until I wondered if Gmail had an API that could be used for my domain's email. Soon after I stumbled upon Gmail for your domain. Not only does it have an excellent interface and a ton of handy tools, but it allows me to download messages in Outlook or to my cell phone, and it also keeps track of mail I send using Outlook. It will also download mail I send via the web next time I open outlook. There are tons and tons of features, but the only downside is they limit # of email addresses as well as inbox size. 2GB isn't a problem at first, but i'm assuming it will fill up eventually.