The NIRVANA Engine is a 'bicolour' (multicolour 8×2) graphics engine written by Einar Saukas, supporting graphics using 22 character rows and 30 character columns. It's open source and royalty-free (even for commercial programs). Further details can be found in this thread. The latest version is available for download here.
It natively supports all standard Spectrum models (48K, 128K, +2, +2A, +2B, +3). There's also an unofficial (partial?) port for Pentagon machines, as described here.
List of released games using the NIRVANA Engine: