SMC is no longer under active development. However, there is now a new game based on SMC:
https://github.com/secretchronicles/TSC
From the official website at http://www.secretmaryo.org:
Secret Maryo Chronicles is an Open Source two-dimensional platform game with a design similar to classic computer games. SMC has computer support to a great degree by using an accelerated Open Graphics Library for the best possible graphic design and stock performance.
It features a wealth of levels, powerups, great music and much more — see http://www.secretmaryo.org/index.php?page=about for a more comprehensive list. And for those still not getting enough, it provides a great in-game level editor.
This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.
This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
You should have received a copy of the GNU General Public License along with this program. If not, see http://www.gnu.org/licenses/.