I'll heartily agree with Kuato!
The x10 MouseRemote using the maX10 software has to be the coolest gadget around. I've been using mine for about 1 year, and could not be happier. In fact, I recently bought a second MouseRemote to have in reserve in case my original one breaks or wears out, I like it so much.
I looked around for a while at many of the other remotes mentioned here, and they are all really limited and expensive when compared to the MouseRemote/maX10 combo. However, the software that comes with the MouseRemote is so lame that if that's what you used, you'd think the thing was the biggest POS you've ever seen.
The MouseRemote is $25, maX10 is freeware, and it works with Win 9x and 2k, don't know about Win Me. You can control ANY software app with this combo, it's super easy to program, and it can also generate on-screen feedback for your commands. It also gives you a pad that can control your mouse cursor. And, as Kuato pointed out, it's a "universal" remote, so it can control your TV and other stuff, too.
Now I sound like a commercial!
EDIT: Subhuman sneaked in while I was writing my original post. The X10 website is certainly a sight (pun intended!) to behold, but who cares? I actually paid for about $50 worth of stuff from them and have received about $150 worth of stuff FREE using their rebates, etc. that he is complaining about! The X10 stuff I have, and it is a considerable amount (over 20 items), is as good as any other mainstream stuff out there, IMO. All my buttons work fine, nothing has broken or worn out, and it is used EVERY day. My only complaint is that after you buy something, X10 tends to send you about 4 e-mails every day, but lots of times it's for free stuff, so it's not a huge ordeal.