I have a 1996 eclipse GST 5spd, and about half of the time when I try to take off in 1st gear, the shift lever pops back and the car slide out of gear. It doesn't do this in any other gear, and I'm wondering what might be the problem.

 

Most likly a syncro starting to go, but first check simple things like shift linkages and such.

 

Ya to me that's the first thing that I thought of, was that the syncro in 1st gear r on there way out. If that indeed is the problem you'd have to get the tranny rebuilt, or at least the syncro for 1st gear, but u might as well have them all done while the trannies out and apart. And usually the syncro's go if u beat on it and slam it into gear, unless the car has over 150k miles on it and its only from normal wear and tear.
And if you wanna buy another tranny, good luck they are so rare to find and usually arent cheap and u'd just end up replacing the syncro's down the road anyways