The GIMP, for one.
But if you can stretch to about £30, you could get a copy of PhotoImpact. I have been using that alongside Corel's software for many years and it is extremely feature-packed and pretty effective. It gives at least 80% of the features of CS3 with at least 70% of the ability, in my opinion. Now that Corel (which now owns it) has apparently decided to market the new version X3 in the US only (it is available from the US section of Corel's website) you may prefer to buy a hard copy of version 12 in the UK. click here