The best place to post is on the WordPress support forums. For example, if you needed help with WP Grins SSL, you’d go to the WordPress Plugin Repository Page, scroll down and look on the bottom right where it says you can post your own topic. Do that and I’ll get notified. I also put a link at the bottom of each post in My Plugins.
Most, basic, break/fix problems I’ll grab ASAP. But if you want an enhancement or something fancy, I’m probably not going to do it unless I think I need it too! This is something I do in my free time, after all, and while I’m not a consultant/expert for hire, I am willing to help people for free on the small things. That’s why I help in the WordPress Support Forums. Please note that there is a point where I will tell you ‘Here’s the basic idea, but you’ll have to sort it out yourself.’ This is specifically BECAUSE I’m not getting paid by you.
If you’re asking for a customization only you use, I generally suggest you fork the plugin. Most of the stuff I write does not require updating, except for new versions of WP (and even then I do my best to make sure I don’t need to do that – Register IP is about the only one that gave me fits). I strive for simplicity.