Trillian Pro has reminder features, and of course, it's a really good messenger which lets you use AIM, MSN, ICQ, YIM and IRC simultaneously. Highly recommended!

http://www.trillian.cc

The downside? The pro version costs $25 USD. The free version doesn't come with any extra skins and plugins, so you couldn't get a reminder feature for that one.

"Stephen Wolfram is the creator of Mathematica and is widely regarded as the most important innovator in scientific and technical computing today." - Stephen Wolfram