Solution to A Tour of Go final exercise - modify a web crawler to fetch URLs in parallel without fetching the same URL twice. Use Go's concurrency features. I used sync.WaitGroup to control the goroutines and a simple string->bool map to store already fetched URLs. Inline comments have further info

crawler.go

package main

import (
	"fmt"
	"sync"
)

type Fetcher interface {
	// Fetch returns the body of URL and
	// a slice of URLs found on that page.
	Fetch(url string) (body string, urls []string, err error)
}

var fetched = make(map[string]bool) // keep track of fetched URLs
var wg sync.WaitGroup               // global WaitGroup

// Crawl uses fetcher to recursively crawl
// pages starting with url, to a maximum of depth.
func Crawl(url string, depth int, fetcher Fetcher) {
	defer wg.Done() // notify WaitGroup  when we're finished
	if depth <= 0 {
		return
	}
	if fetched[url] == true { // only need to fetch each URL once
		return
	}
	body, urls, err := fetcher.Fetch(url)
	fetched[url] = true // mark it fetched
	// NOTE: If above is placed after the err check then not-found URLs will be
	// tried to be fetched again the next time they're encountered, having it
	// before means that once they've 404'd once, they're considered done with
	if err != nil {
		fmt.Println(err)
		return
	}
	fmt.Printf("found: %s %q\n", url, body)
	for _, u := range urls {
		wg.Add(1)                     // notify WaitGroup of a new goroutine...
		go Crawl(u, depth-1, fetcher) // ... and dispatch it
	}
	return
}

func main() {
	wg.Add(1) // add initial goroutine to WaitGroup
	go Crawl("http://golang.org/", 4, fetcher)
	wg.Wait() // await completion of all goroutines
}

// fakeFetcher is Fetcher that returns canned results.
type fakeFetcher map[string]*fakeResult

type fakeResult struct {
	body string
	urls []string
}

func (f fakeFetcher) Fetch(url string) (string, []string, error) {
	if res, ok := f[url]; ok {
		return res.body, res.urls, nil
	}
	return "", nil, fmt.Errorf("not found: %s", url)
}

// fetcher is a populated fakeFetcher.
var fetcher = fakeFetcher{
	"http://golang.org/": &fakeResult{
		"The Go Programming Language",
		[]string{
			"http://golang.org/pkg/",
			"http://golang.org/cmd/",
		},
	},
	"http://golang.org/pkg/": &fakeResult{
		"Packages",
		[]string{
			"http://golang.org/",
			"http://golang.org/cmd/",
			"http://golang.org/pkg/fmt/",
			"http://golang.org/pkg/os/",
		},
	},
	"http://golang.org/pkg/fmt/": &fakeResult{
		"Package fmt",
		[]string{
			"http://golang.org/",
			"http://golang.org/pkg/",
		},
	},
	"http://golang.org/pkg/os/": &fakeResult{
		"Package os",
		[]string{
			"http://golang.org/",
			"http://golang.org/pkg/",
		},
	},
}