He's not still releasing albums, his mom is still releasing them through the company Tupac established before he died. Tupac was a workaholic so to say, he worked long hours and into the wee hours of the morning everyday dedicated to creating new songs, music videos, as well as on the set of his movies. More albums have come out after his death than when he was alive, and not only that but these albums are almost all double albums containing over 25 songs per album. There were also 2-3 music videos which came out after his death (maybe just two of them) as well as two movies which he had starring roles in.
Based off the many interviews of Tupac and the content of his songs, Tupac always saw himself dying at an early age and knew he didn't have the luxury of time. Especially after he was shot 5 times and lived, he was convinced he only had so much time so he created a body of work that would continue his legacy.
A lot of people, especially those who don't listen to hip-hop at all, completely disregard the magnitude of his work. Personally I've found its been because of racism and supremacy (not necessarily white supremacy), plain and simply there are those out there that consider it black/ghetto music that doesn't relate to them. Honestly, I'm not a big fan of rap, I enjoy the art (I enjoy poetry and rap can be very similar) but for the most part I don't like many of the artists and their work. However, if you give the music a chance and take in the scope of Tupac's work from beginning to end, its pretty mind-boggling. Even if you aren't into his music, his story is just as entertaining because it is interlaced with so much mystery, puzzlement, contradictions, not to mention "clues" about his death that seem to be too obvious to be a coincidence.
If you can look past the "blackness" of his work (I don't mean to use this term and sound racist, I just use the term to describe his use of the word "nigga" so often as well as his references to the black community) you'll find a lot of intriguing meanings.