This virtual manipulative allows you to solve simple linear equations through the use of a balance beam. Note: it is a large file, so it may take some time to load.
http://nlvm.usu.edu/en/nav/frames_asid_201_g_4_t_2.html?open=instruction...

Algebra Balance Scales - Negatives
This version includes negative numbers and negative coefficients on x.
http://nlvm.usu.edu/en/nav/frames_asid_324_g_4_t_2.html?open=instruction...