Up until April 2011, citizens from the Czech Republic, Estonia, Lithuania, Latvia, Hungary, Poland, Slovakia, and Slovenia were required to partake in the Worker Registration Scheme if they wanted to work in the UK, due to transitional arrangements regarding the respective countries joining the EU.
Although the Worker Registration Scheme has now ended, employers who wish to take on workers from Bulgaria and Romania - which joined the EU more recently than the countries noted above - need to apply for a Work Permit. There are six types of Work Permits and there are some exceptions to the requirement. More information can be found on the UK Border Agency website (see link below).
UK Border Agency - Work Permits