Sycamore is easy to turn, and cheap(ish), Ash is good as Chris has already mentioned, Beech is also a nice wood to turn and is reasonable, or I have recently discovered London Plane is a dream to turn, and again is not expensive, and has a very intersting figure to it.
Most native woods would fit the bill, with the possible exception of Oak, which I find a bit more difficult to turn, and it blunts the tools quickly!
Also worth having a try of some green wood, you can usually get it free from tree fellers or your local woods. Need to be patiant and wait for it to dry after rough turning or learn to turn it very thin so it dries as you turn. It is also much easier to cut.
If you are looking to practice purely to develop your skills I would recommend using a softwood such as pine. Because it is a softwood the grain will tear very easily. Your chisels/gouges need to be sharp and then when you can produce a good finish on your coves/beads/fillets, etc. on a softwood you will know that you are using the tools correctly. You will then be able to apply the same principles to any wood with satisfactory results. Pine is cheap enough (and often found in builders' skips) to practice on and then discard. Save the expensive hardwoods for making things when you have your skills - plenty of practice and it won't take you long :wink: .
Bought some bits and pieces at a local timber yard at the weekend. The guy had these spindle blanks almost as scrap. bought about four meters of hardwood for 10 euro's. Not too bad and good for practice too.
I like the idea of using pine for skills though, didnt think of it in those terms.