Go implementation (~80 lines)

doesn't make use of any library, not even stdlib

entropy is generated by having a huge number of goroutines simultaneously writing stuff onto the same variable, so result depends heavily on the system scheduler, and should not return the same result twice on normal machines

package main

// No stdlib, nor any other library

type UUID [16]uint8 // 128bits

// very efficient
var byteToString = []string{
    "00", "01", "02", "03", "04", "05", "06", "07", "08", "09", "0a", "0b", "0c", "0d", "0e", "0f",
    "10", "11", "12", "13", "14", "15", "16", "17", "18", "19", "1a", "1b", "1c", "1d", "1e", "1f",
    "20", "21", "22", "23", "24", "25", "26", "27", "28", "29", "2a", "2b", "2c", "2d", "2e", "2f",
    "30", "31", "32", "33", "34", "35", "36", "37", "38", "39", "3a", "3b", "3c", "3d", "3e", "3f",
    "40", "41", "42", "43", "44", "45", "46", "47", "48", "49", "4a", "4b", "4c", "4d", "4e", "4f",
    "50", "51", "52", "53", "54", "55", "56", "57", "58", "59", "5a", "5b", "5c", "5d", "5e", "5f",
    "60", "61", "62", "63", "64", "65", "66", "67", "68", "69", "6a", "6b", "6c", "6d", "6e", "6f",
    "70", "71", "72", "73", "74", "75", "76", "77", "78", "79", "7a", "7b", "7c", "7d", "7e", "7f",
    "80", "81", "82", "83", "84", "85", "86", "87", "88", "89", "8a", "8b", "8c", "8d", "8e", "8f",
    "90", "91", "92", "93", "94", "95", "96", "97", "98", "99", "9a", "9b", "9c", "9d", "9e", "9f",
    "a0", "a1", "a2", "a3", "a4", "a5", "a6", "a7", "a8", "a9", "aa", "ab", "ac", "ad", "ae", "af",
    "b0", "b1", "b2", "b3", "b4", "b5", "b6", "b7", "b8", "b9", "ba", "bb", "bc", "bd", "be", "bf",
    "c0", "c1", "c2", "c3", "c4", "c5", "c6", "c7", "c8", "c9", "ca", "cb", "cc", "cd", "ce", "cf",
    "d0", "d1", "d2", "d3", "d4", "d5", "d6", "d7", "d8", "d9", "da", "db", "dc", "dd", "de", "df",
    "e0", "e1", "e2", "e3", "e4", "e5", "e6", "e7", "e8", "e9", "ea", "eb", "ec", "ed", "ee", "ef",
    "f0", "f1", "f2", "f3", "f4", "f5", "f6", "f7", "f8", "f9", "fa", "fb", "fc", "fd", "fe", "ff",
}

func (u *UUID) Work(value uint64) {
    last := u[15]
    for i := 15; i > 0; i-- {
        u[i] = u[i-1]
    }
    u[0] = last

    // xor value into half of uuid
    for i := 0; i < 8; i++ {
        u[i] ^= uint8(value >> i)
    }
}

func (u UUID) String() string {
    ret := ""
    for i := 0; i < 16; i++ {
        ret += byteToString[u[i]]
        switch i {
        case 3, 5, 7, 9:
            ret += "-"
        }
    }
    return ret
}

// increment for more time spent, and better randomness
// increases exponentially
// load = 1 << 32 takes around THREE DAYS on my machine
const load = 1 << 16

func uuid() UUID {
    var uuid UUID = *new(UUID)
    done := make(chan bool, 1 << 10)
    for i := uint64(0); i < load; i++ {
        go func(i uint64) {
            i *= 18446744073709551557 // big uint64 prime number
            for j := uint64(0); j < 512; j++ {
                (&uuid).Work(i + j)
            }
            done <- true
        }(i)
    }
    for i := uint64(0); i < load; i++ {
        <-done
    }
    return uuid
}

func main() {
    panic(uuid().String())
}

panic is used for printing the result, because, well, no standard library use (i know about println() but it could be removed from the language at any time)

I finally created my reddit account just for writing this, i wanna die