Crawler applications

Xenu Link Sleuth (free)
Analog X Link Examiner (free)
Screaming Frog (paid)
Integrity (For MAC – free)

Scraper applications

Scraper Extension for Chrome
Scrapebox (paid)

Link Intelligence software

Open Site Explorer (free & paid)
Majestic SEO (free & paid)

HTTP Analysers

HTTP Fox (Firefox)
Live HTTP Headers (Firefox)

IP checkers

Show IP (Firefox)
WorldIP (Firefox)
Website IP (Chrome)

Link checkers

Simple Links Counter (Firefox)
Check My Links (Chrome)

Monitoring tools

Uptime Robot (monitors domains for downtime)
Robotto (monitors robots.txt)

Rank checkers

Google Global (Chrome)
SEO SERP (Chrome)
SEO Book Rank Checker (Firefox)

Site performance analysis

Yslow (Firefox)
Page Speed (for Firefox)
Page Speed (for Chrome)
|Speed Tracer (Chrome)