http://www.clarkconnect.com/(one i'm using, get the commnunity edition(free), not the paid version), http://www.ipcop.org/, http://www.ipcop.org/ or netlimiter if you using windows xp. You need atleast 2 network cards. Works like a charm because i'm sharing 10 pc's for 1 internet connection. The pc i use pentium 3 with 256 sdram to serve this purpose. For instructions & guides please refer those respective websites i lazy teach so much. One more thing this is for who have good experience with computers.(ehem basic networking knowledge.......) 
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