It can either be relaxing or torture, depending on what you are going for. :D
We have this: http://www.lovehoney.co.uk/product.cfm?p=26082 which is great for more relaxing tickling. Slowly running it over our bodies gives us shivers and goosebumps, blindfolded is even nicer as you don't know where to expect the feathers next. It is teasy, soft and lovely.
Then for more tortururous tickling you have fingernails, among other things. (Don't rule out the intensity of whispering into someones ear or kissing their neck or ears. Blowing gently or licking here) There is nothing quite as intense as being bound up, unable to escape and feeling that panic as his hands or lips trace over my ribs or neck (I don't have very tickly feet or armpits....but go near my neck and I am screaming lol) Not being able to stop it because of the restaints is....intense. I like it, only in short bursts though. It is very physical (All that jumping around and laughing til your ribs ache lol) It is like a panic, a loss of control.
Just make sure your partner is into it and talk about any limits beforehand. Everyone is different, I know some HARDCORE masochists who can take being whipped until their ass is blue but tickling is a hard limit! Some absolutely love it but maybe only the first scenario where they have more control or it is less intense. Some have a streak in them that makes them crave that panic/out of control feeling of the more intense play. Everyones different x