After having used MarketPress for several months, I've written several add-ons, or helpers, and I'm making them available to the community. These are all developed for MarketPress ver. 2, so make sure you upgrade first!
If you use these add-ons and like them, please consider clicking the "Say Thanks" button above. I'm trying to get to 1000 points.
1. MarketPress Grid Layout Shortcode
This is a little plugin that adds a shortcode called [mp_product_grid]. It has all the same attributes that the original [mp_list_products] shortcode has.
(1) Unzip and drop the /marketpress-grid/ folder into /wp-content/plugins/
(2) Activate the plugin like any other in WordPress
2. A new MarketPress Store Theme
This MP theme uses the same CSS3 buttons/styling that the guys from WooThemes use (great for anyone who's using a WooTheme with MarketPress). You can see it in action here: http://thebutterflycircus.com/store/
(1) Unzip and drop the CSS file (and the folder) into the /marketpress/marketpress-includes/themes/ folder
(2) Visit Store Settings >> Presentation inside your WP dashboard. The "Skyrocket" theme should show up in the dropdown box.
3. A new flat-rate shipping plugin
This MP shipping plugin allows more options for shipping internationally. Currently, there's only 1 box to add extra shipping per item. This plugin allows for more options -- handy for when shipping prices vary wildly between different countries. More info in this thread.
(1) Unzip and drop the PHP file into the /marketpress/marketpress-includes/plugins-shipping/ folder
(2) Visit Store Settings >> Shipping inside your WP dashboard. The "Flat Rate Intl" theme should show up in the dropdown box.
It's my hope that in case these add-ons become popular that they might be wrapped into the core of MarketPress eventually.