Video editing tools come and go, but one stalwart in a sea of applications in VirtualDub. The free AVI editor continues to be a handy utility, even with the rise of better container formats such as MP4 and MKV (which VirtualDub doesn't support). If you have an AVI you want to quickly convert to a GIF for use online, VirtualDub can help.
It honestly couldn't be easier. Just open the AVI file you want to convert, then go to File -> Export -> Animated GIF. You'll then be able to choose the export filename and the loop type of the GIF.
I wasn't expecting much in terms of quality — GIF only supports 256 colours, so going down from 16.7 million you should expect a loss in fidelity — but the resulting file was quite good. Of course it depends on how complex the colours are in the source video, but for simpler clips, it'll do just nicely.
VirtualDub [Official site]