Greenshot is a great alternative for SnagIt. SnagIt is a great screen capture tool and I highly recommend it for a commercial enviornment. But it’s one of those tools that is nice having it on a home system that you are doing sidework. Greenshot is open source and has an executable that installs on any Windows platform. Its a basic set of tools similar to SnagIt to perform screen capture operations; but if you are looking for pretty advance features stick to the commercial leader SnagIt.
I have listed this tool under the RESOURCES web page on my blog.