I inherited a beautiful Monsoon cardigan from Zoë at the club clothes swap a few months ago. It fitted me really well except that the sleeves were too short for my beanpole arms (I get this a lot!). Luckily, the cardigan had a crochet-effect trim down the front, which inspired me to add an inch or so to the arms by making something which matched.
I simply crocheted a chain as long as the circumference of the sleeve and made it into a ring. Then I did two rows (or whatever you need) of double crochet, and then a row of shell-shaped stitches. I got this from a book but there are plenty of tutorials on the web – for example this youtube video (remember that Americans have different names for crochet stitches!). I’m very pleased with the result and it makes the cardigan a lot more wearable 🙂