Charter flights from all over Europe as well as scheduled flights from Madrid, Barcelona and Valencia.
Most people have their flight included in a package holiday, but its becoming increasingly common to book on many of the 'No Frills' charters and arrange you own accommadation.
Check these links for cheap bookings from the UK.
For flights from the Spanish mainland.
If you have a lot of time, and a cast iron stomach, you might chose the ferry to bring you to Ibiza. But, for only a few more dollars/euros, you can get a cheap flight from Barcelona (BCN) to Ibiza (IBZ). I paid 50 Euro for the no frills flight, and it puts you only a 10 minute cab or bus ride from the center of Ibiza Town. Also, when you first arrive at the airport, MAKE SURE you look around for the club people. They are handing out free stuff, entrance tickets, drinks, etc. Also, get your map of the island here, as it is the best one.
Plenty of cheap flights from UK to Ibiza... If you're sharing a room then a package including accommodation can be the cheapest way.. but I find for single travellers booking flights and accommodation separately gives you more flexibility and is cheaper if you can be bothered to put the time in doing a bit of internet surfing