Todoist, Things, ToDoList, TickTick, Super Productivity, Wekan, Taskade, Org mode, Vikunja, Taskwarrior, Task Air, TeuxDeux, GTasksTo-Do List, SwissTasks, Quire, NowDo, Condution, ZeroTodos, Tasks.org, Task Coach, Remember The Milk, Top Task List, Gtdagenda, Go For It, Chaos Control, Zanshin, iGTD, Week Plan, Lifestyle Inspector, Nullboard, Nirvana, Everdo, EfficientPIM, UV Outliner, Cheddar, Calcurse, iMDone, Agenda At Once Free PIM, Zettel Notes, Hyper Plan, todotxt.net, Getting Things GNOME, TodoWiz, TaskLite, Tasktop, Taskary, Pi Reminder, Todour, Todoist Wall, Tasque, Simpletask Cloudless, Actiontext, TodoListMe, TodoPaper, Nozbe, DoItRight, Firetask, Wieldy, Nibras PKM, Five.Today, quuxTodo, TaskFabric Cloud Projects, Business task, GeeTeeDee, FCorp My Calendar, QTodoTxt 2, Milkify, ccToDo To-Do List, topydo, Taskastics, Trim Milk, Todoey, Eggscellent, DoTimely Hub, WinMilk, SomTodo, Remember The Task, Mydoid, DoItNow!, Todo.EXE, dstask, Kalendar, Gnome To Do, MonoCorpus, Kocpit, Taskline, MyMaelstrom, Yokadi, Taskian, Black Hole Organizer, Qiknr, NoProb To-Do List, Alinof ToDoList, DevTodo, GTD Simple, ReminderFox, DayTasks, TaskAgent, jdotxt, todo.pink, Domestica, Quicktask (Vim plugin), Mirakel, Effitask, SwiftoDo, Got to Do are some alternatives to Sleek.