Working on my abs?

I recently got a membership at my local gym, and I'm happy using most of the machines, but one area which I would like to tone is my abs, and my gym only has one abs machine (Which I could never work out how to use properly without hurting my back...) and it always seems to be broken! I've tried sit ups but am not very good at them and they're so boring! :( What does everyone else do to tone their abs, is there any low-cost equipment I could buy, for example?

I recommend bicycle crunches, which I do on a regular basis and work very well for abdomens.  You can see videos online on how to do them.   They do not require any machine.  There are other options.  Look at this article, too:

Your abs function to support your body.  The best exercises for it are the ones that require your abs to do that support.  Full body heavy lifting-will work your abs hard, so will planks, and running, especially sprinting intervals.

Get off the machines and do free weights and a full body lifting routine - that will not only work the entire body but force you to use your whole core to stabilize your body.

I am currently doing the program "New Rules of Lifting for Abs" and there is no machines except cables for chops, but all the other exercises are all free weights, lots of variations of planks. Deadlifts, squats, overhead presses, very little "sitting down" lifts.  Standing up works your body from head to toe.  

I haven't done a crunch or sit up or anything like them because they really do not improve abs and can cause injury to back.  do planks, side planks, elevated planks work on building up to a 2 min plank- try planks with one hand held up, one leg, one hand one leg- there was also a plank row using the cable machine and planking and rowing  - farmer walks, turkish get ups, cable chops, there are so many things that can be done machine free that will improve your entire body.

HIIT and Tabata sprints!!!

and weight training followed by 20-30 minutes of light-moderate cardio

smashley is right your abs work best when you work them along with another muscle group. the best is to do heavy squat and deadlifts this will strengthen and shape your abs and actually ALOT better than ab crunches :)

Thanks for the suggestions! I didn't realise running toned abs! I love running :) But I do it a lot and still have a flabby stomach :( I'm not too keen on free weights but I'm going to go try out some of those exercises off the linked article, and squats and planks etc! :)