I have a computer that needs to connect to my modem...

I have another computer and I need to get internet access to it. Is there some kind of device that lets you plug multiple RJ-45 cables into one device and that device can plug into a cable modem? I don't want to spend another $30-$50 on another cable modem or get a wireless router.

Yes there is.

http://www.amazon.com/NETGEAR-RP614-Web-Safe-Router/dp/B00007E6Z7/ref=sr_1_3/002-1664323-3668850?ie=UTF8&s=electronics&qid=1191974569&sr=8-3

This will let you plug an internet router in via RJ45 and connect up to 4 computers also via RJ45. It's also a hardware firewall and really easy to set up. I got one and it made life a lot easier for internet sharing. It's also very stable, I've never had a problem with it.

Have fun.