I second getting a switch. They take one port and branch it out to several ports. Most are plug-and-play, just be sure to only connect one cable to the switch and the router. Two or more connections create loops. Same with connecting the switch to itself; just don't do that.
As far as CAT5e switches/splitters are concerned, they do sell these, but yes they are problematic and I don't even know why they are on the market. Switches can cause handshaking and negotiation issues. Splitters will reduce your maximum bandwidth to 10Mbps up and down, or less depending on whether or not you manage to get Full Duplex or not. If one of two pairs of the two bundles in a CAT5e cable being split are bad, one PC will not get a link what so ever.