For under 100 dollars you're not going to find anything really great. I'm currently rocking Altec Lansing speakers that have the mid drivers pointed down at the desktop. It kind of muddies the mids but it makes the soundstage pretty expansive. I like it. Especially since they have 4 tweeters pointed at my head, so the highs are nice and clear. For under 100 dollars, Altec Lansing makes some pretty good arguments, but the safe choice in computer speakers is the Klipsch Promedia 2.1, which are roughly twice what you want to spend.
The Gigaworks T40 seem promising - I haven't heard them myself but I don't find too many negative reviews on them so they would be on my list of competitors. However, they run typically over 100 bucks, and there's no subwoofer option. I prefer having a sub to get more bass extension but with computer speakers, that usually means that some of your lower mids are going to be coming out from under your desk (if you turned the "treble" switch all the way down, you'll hear some vocals coming from the sub.)
I see fewer reviews on the T20, but the T20 is basically a T40 with two fewer 2.5" drivers. I'm always a fan of more drivers over fewer for computer speakers (mine have two tweeters per satellite... ) since the drivers themselves are so small, but four 2.5" drivers are hardly necessary.
Bose Companion - wholly unnecessary spending and you're getting a single 2.5" driver per satellite and no tweeter. They're supposed to have some kind of special DSP that makes the soundstage large and expansive but I didn't notice anything like that. I don't even think they look that nice.
