I despise LinkedIn however it's still the largest database of professionals out there so I pay for a 'recruiter lite' account which is quite useful.
connectifier.com/search is great for saving time in trying to extract an individuals contact info.
Gild.com is insanely expensive but a decent investment if you're hiring a significant number of people.
gender-decoder.katmatfield.com is a wonderful (free) resource to help you ensure your job adverts are using universally appealing language.
charliehr.com is a great, lite-HR system that allows you to manage all of your documentation, holidays and payroll details in one place, and it's free.
twiangulate.com/search/ is quite useful for discovering potentially interesting people on twitter.