I've done quite a few. It depends on what section of IT you want to get into. My work has paid for all mine, as I already worked for them in a different role before I got this job.
For networking what I do, the best course is CCNA (Cisco Certified Network Associate). This is a very good grounding to get into the sort of job I've got, and you can command £30k to start with. Then, it depends on what sort of company you want to work for, as to what other stuff you can do.
Desktop support - not sure what courses, as desktop support will depend on what company you want to work for.
The thing I like about my job is it's really varied. It's not the usual IT stuff......"My computer won't turn on" etc. My company routes transactions.....credit card authorisations, and also traffic for the stock exchange, and a few other bits and bobs, and if a leased line which the data is being passed on goes down, companies can loose £1000's.
