If you want to insert few images to a post, then native Gallery feature of WordPress works just fine. But if you have lots of images to post, then obviously you would want to improve upon the basic gallery with few additions to improve the over all photo viewing experience. Here I will list some plugins which you can use to improve upon the existing gallery features with which you can have a professional Gallery on WordPress without much effort:
The native WordPress gallery shortcode generates XHTML Invalid code. This plugin will help you fix that. But this is not the only feature that Cleaner Gallery provides. It allows you to add multiple galleries per post, allows editing the number of pics per gallery, selectively loads JS/CSS for your gallery and supports 18 Lightbox Effect plugins to display your photos.
Default Gallery shortcode shows all photos on a single page. For 10-15 photos, this works fine but not if you have lots of images in a gallery. This plugin breaks the gallery page into multiple pages with a configurable number of images on each page. A must have plugin if you are planning to make a Gallery type site with Native Gallery feature only.
When you have lots of images to share, then uploading them via WordPress Flash Uploader can become cumbersome. To solve that problem, WordPress Flash Uploader was created. You can upload your images via FTP and WFU will sync them to your WordPress’s Media Library. You can insert then those images to your WordPress Gallery. WordPress Flash Uploader can be accessed via its own menu or via the upload tab inside Post Editor.
The name of the plugin suggests that it adds a lightbox type effect to the Gallery images. But it does lots more than that. It displays the associated metadata of the image, adds pagination to the galleries, extends the gallery options and can change the caption view of images to tooltips. If you are adding to spice up your gallery with some effects, this is the plugin you should go in for.
There are other Lightbox plugins as well you can use. Basic functionality is same but they offer various extras. Its upto you which one suits your needs: jQuery Lightbox for Native Galleries, Flexible Lightbox, Lightbox Plus, WP jQuery Lightbox and Simple Lightbox.
This is by no means an exhaustive list. There might be dozens of other plugins apart from which I listed. I just listed those ones which I found useful and easy to implement.