Flickr - Free vs. Pro

Free:

• 100 MB monthly upload limit (5MB per photo)
• 3 sets
• Photostream views limited to the 200 most recent images
• Post any of your photos in up to 10 group pools (more on this later)
  
• Only smaller (resized) images accessible (though the originals are saved in case you upgrade later)

Pro:

• Unlimited uploads (10MB per photo)
• Unlimited storage
• Unlimited bandwidth
• Unlimited photosets
• Permanent archiving of high-resolution original images
• The ability to replace a photo
• Post any of your photos in up to 60 group pools
• Ad-free browsing and sharing