I have been working with ArcGIS Server 9.2 lately, it's come a long way since the first version. It's faster now and has a lot of out-of-the-box Ajax features. I have also been working with MapGuide Open Source, it's a great product for something that is free. I had a little experience working with UMN MapServer as well. I plan to continue working with these products as well as some of the mapping APIs (Google, Virtual Earth, Yahoo!, others) and write about my experiences on my website over the coming months.
