If you just want to extend your network you can use your existing electrical system.
This is called power line ethernet You get various version- The simplest being point to point. and more complicated ones that allow for mulitplexing/switching across several units like the one in the image below.
And you can get very advanced and slick ones called power line sockets that are switch traffic across your whole house with these stylish wall plates...
Speeds vary from products but usually start at 55mbs (55megabits = 5mega bytes per second) and I have seen speeds reaching up to 500mbs (50megabytes per second)
Cable is ALWAYS allot more reliable especially if you want to use it for you home multimedia systems, like streaming HD movies from a NAS, playing world of warcraft with ultra low latency so you don't get owned by a noob because of lag. Wireless is good for watching youtube on your smartphone, placing orders from your fridge or letting your guests leach some of your bandwidth. (Trust me- I had Wirless N on my media centre that was 3 meteres above- put in a 100mb cable and now i can watch movies without stress)
Obviously running LAN cable is the cheapest option - I would suggest using some cable conduit.You get various sizes and shapes - and you paint over plain ones if you want to camouflage them into your wall.
But be careful not to land up with something like this.. it is easy.
Some technical benchmarking and limitations
Some more reviews on the netgear 500mbs reviews