I know that the nivo slider plugin: http://nivo.dev7studios.com/features/#wordpress allows you to insert slideshows anywhere you like by using shortcodes. You can create as many slideshows as you like, use different dimensions, etc. You can add the shortcode by using a text widget in the sidebar. To allow shortcodes in sidebar widgets, simply edit the functions.php file from your them and add the following code:
Once you saved the file, you can now add as many shortcodes as you want in sidebar widgets.
I haven't used a nextgen gallery slideshow yet so I'm not sure if there's an alternative out there. I've found: http://wordpress.org/extend/plugins/easy-nivo-slider/ but it seems to be broken and I haven't tested it with our themes.