Ryanair have another sale on, with the offer flights being £5 including tax
They've also tweaked their site: flights that are on offer are now clearly marked as "SPECIAL OFFER" in the booking system.
Booking until Monday
I've checked availability from Manchester (my local airport) and found offer flights to Bremen, Marsaille and Milan - dates below.
outbound - 27/05, 29/05 31/05 03/06, 05/06
inbound - 03/06, 07/06, 10/06, 12/06
outbound - 05/06, 07/06, 10/06, 12/06
inbound - 12/06, 19/06
outbound - 04/06, 11/06
inbound - 10/06, 01/07
Edit: just also found Liverpool - Venice, Liverpool - Stockholm and Stanstead - Genoa all on the offer (all checked on the first 3 weeks of june)
These offers are good for the end of May, all of June, start of July ('till the 15th) and then all of september, so happy hunting!