I'm working on L1B visa in US. Another company wants to hire me but the H1B cap for this year is already over. What Next?

Not much, under your current circumstances you cite. Your years under L-1B will be deducted from the total H-1B time given you, if any h-1B granted in the future. For now remain with your current employer.

Unless your new employer is cap exempt they will have to wait for April 1 st to file a petition for. Assuming such a petition is selected in the lottery you will eligible to work on it in October of 2015. if you find an alternate part time employer that is cap exempt to file a petition for you your new cap subject employer can file a concurrent petition for you without being subject to the cap. Alternately if you have a masters degree and are not born in india or china 0r are married to someone not bornin either of these countries, you can get them to start the labor certification process for you and join employment once the labor is approved after filing the I-140 and 1-485 premium process and receiving EAD. If you are a Canadian air Australian you can get a TN or e-3

Unless they are "cap-exempt", they will have to wait until April 1, 2015 to submit an H-1B petition on your behalf. Please see