The latest Core Player 1.30 has built in support for streaming youtube videos! And, you can even play FLV videos stored on your memory stick too! Yeah!!! Go get it from imserba forums. For streaming YouTube videos smoothly you need at least 3G or WLAN to be working really fast.
Another cool app has been released by CPS(CellPhoneSoft) for P1i and the like(M600). Ever missed the D-Pad on your P1i? Padissimo fills that void!!! Download and install it from here. Launch it, then just long press 0 to hide the screen d-pad. Now use 2, 4, 6, 8 and 5 keys as d-pad anywhere in menu or web browser. To disable d-pad(especially recommended when you are on opera mini), just long press backspace key.